According to the state of the art, as far as extracting and processing a given ore or a mixture of ores is concerned, fines from mines that cannot be directly fed into metal ore production furnaces are set apart for pelletization processes. In a typical pelletization process, these ore or mixtures of ore fines are subjected to a preliminary process through which their granulometry become even finer as they are either ground with fluxing agents or subjected to separate dosage and, lately, are subjected to a binder dosage aiming at agglutinating the particles.
Pellets are made by taking this previously homogenized mixture with adjusted moisture and subjecting it to the pelletization process using pieces of equipment that are known to the state of the art, which are often called pelletizing discs or pelletizing drums in which microfine particles are agglomerated to form pellets (usually called crude or green pellets), partially spherically shaped with medium diameter, as required for use in subsequent industrial processes.
Further, these pellets are then classified and fed into a heat treatment or sinterization furnace for induration.
During handling, inside the pelletization disc and during the loading process into the heat treatment or sinterization furnace, it is known that the green pellets are oftentimes damaged due to a number of factors such as the distance they have to cover, the height and number of falls they are subjected to, the speed of the transfer belts, counter-flow transfers, and many other factors.
At the end of the sinterization process, these pellets are furthermore classified for the removal of fines, and fired fines-free pellets are eventually used in subsequent industrial processes. Typically, in the case of iron ore, fired pellets are commonly used in the production of pig or sponge iron, both consisting of raw materials employed on the production of steel.
Within the above described process, the pelletization disc comprises a metallic disc or circular tray fitted with a rotary movement in the inclined plane and scraping devices that favor the formation and growth of seeds by means of rolling and binding motions, in addition to the incorporation of particles until a pellet-shaped product is obtained, while the ore is fed into the disc. As variables are adjusted over the course of this process, the goal is to secure an improved sphericity, within the desired granulometry specification, in addition to the intended diameter for pellets within a most favorable productive range for use in subsequent industrial processes.
Nevertheless, one of the inconvenient factors of the state of the art is that the continuous loading of ore and the continuous scraping process carried out at the bottom of the disc or tray, along with other mechanisms, end up contributing to a final product containing significant quantities of fines and also to pellets comprised outside the desired size range, which can amount to over 20% of the total mass of the material. This problem gets even worse when the disc or tray is replaced with a pelletization drum, which, by nature, holds a very high degree of recirculation load, which is equivalent to the percentage of below and above a certain particle size range that is routed back to the fragmentation and pelletization process, and can amount to up to 50% of the total mass of the material.
Another inconvenience of the state of the art is the difficulty in obtaining pellets with adequate sphericity degree. This is due to the fact that several mechanical and physical complex processes, already known by the state of the art, occur simultaneously during the time pellets are forming and growing in an environment containing a large mass of material. Among complex pelletization processes, nucleation, coalescence (or fusion) and stratification (see FIG. 1) stand out.
These mechanisms are adversely affected by various sources, including the action of both bottom and side scrapers, which are common in pelletization equipment, and that redirect the flow of pellets being formed. Disc inclination and rotation speed, as well as feed ore moisture, and the production itself are also factors that influence the quality of pellets. Furthermore, low porosity plays an important role in the resistance of the agglomerate and, therefore, should be obtained prior to the heat indurating process.
Another inconvenience of the state of the art is the difficulty in ensuring an appropriate and homogeneous compactness and organization of the ore grains that make up the pellets, leading to pellets friable points or internal areas, which are conducive to the generation and propagation of cracks as pellets are transported to the furnace. If, on one hand, the rolling motion time is fundamental for such compaction, on the other hand an excessive speed developed by pellets inside the discs may lead to a crack formation process in case these pellets collide with the disc sides.
Another inconvenience of the state of the art is the difficulty in ensuring pellets with a lower degree of roughness in relation to its surface finishing, thereby making them coarse and predisposing them to the generation of fines through abrasion during their transportation to the heat treatment or sinterization furnace, in addition to the generation of dust as they are moved after being fired. This, too, is due to the various simultaneous processes used in pellet formation, including ore feeding rate and moisture.
In order to allow the elimination or reduction these hindrances, various control methods have been proposed for the pelletization process, including the variation of parameters such as moisture, amounts and types of binding agents, rolling motion time, mass proportions and size distribution of the used fines, with each method carrying its own disadvantages.
The time and the conditions available for produced pellets to show a more spherical shape are not enough in conventional pelletizing discs or drums. Hence, if the rolling motion time is increased for these devices while being concomitantly fed with ores, and owing to the mechanisms for shaping the crude pellets, the average size of such pellets increases without the occurrence of the corresponding appropriate sphericity, this being one of the identified disadvantages.
The state of the art also comprises the description of multiple-stage pelletization processes. However, oftentimes some of the disadvantages of such processes are the need to interrupt the processing flow due to the inclusion of additional phases for transporting and reloading pieces of equipment or the need for the assembly identical large-size equipment series or circuits, thereby leading to burdens associated with the use of space and resources.
Therefore, notwithstanding the control methods predicted by the state of the art aimed at improving pelletization processes, there remains in the state of the art the need to overcome the problems associated with these processes in order to obtain more compact and homogeneous ore or ore mixture pellets, without increasing the rolling motion time or volume, ensuring a decreased generation of fines, and using a fewer number of stages and less complex equipment.
Surprisingly, the present invention discloses that the use of a two-stage pelletization process in which an additional treatment stage following pellets generation and the preliminary production in pelletizing discs or drums result in an improved physical quality of crude ore or ore mixture pellets, thereby mitigating the inconveniences of the state of the art.